Wednesday, March 15, 2017

How to prevent a roof collapse during a massive snowstorm

March 14,2017, 2:26:05PM,EDT

When a major winter storm unfolds, home and business owners have more than messy roads to worry about.
Heavy
snow can pose a serious risk to roofs and cause some to collapse,
creating dangerous conditions. Still, experts say the safest way to deal
with piles of snow is to call for professional help.
AccuWeather
Forensic Meteorologist Steve Wistar said home and business owners
should hire contractors to remove the snow or use rakes if the roof is
not too high.
In the northeastern United States, roofs are
generally designed to support 30 pounds per square foot, Wistar said,
but some roofs are built to support 40, 50 or even 100 pounds per square
foot, according to the ASCE Standard Handbook.

Bong Wang uses a snow
rake to try and clean an ice jam from the roof of his home, Saturday
Jan. 9, 2010, in Omaha, Neb. (AP Photo/Dave Weaver)

“But 30 pounds per square foot is just a starting point, then you have to factor in the other variables,” Wistar said.
The amount of snow accumulation is not always the best guide to determining how much weight snow is putting on a roof.
"If
the roof is sagging, it has to be handled very carefully because the
added weight of people shoveling the snow could cause a problem," said
Anthony Maiocco from Maiocco Structural Engineering. "And a contractor
would have more experience with safety." Weight of snow
The
inches of snow accumulation do not directly dictate the weight that is
placed on a roof. The weight of snow is actually dependent on the water
content, Wistar said.
One inch of water weighs 5.2 pounds per
square foot, and snow can have different distributions of water content,
which depends on a number of variables including outside temperature.
Wet snow weighs more than dry snow because it has a higher water content.
Dry,
powdery snow weighs less and its dusty, flaky texture makes it prone to
drifting, which is ideal for any roof designed for drifting.
But
roofs with external roof units, such as air conditioning or parapet
units, are more prone to uneven weight distribution as the snow drifts
against these units instead of blowing off the roof. When powdery snow
is blown in the wind, the snowflakes break apart and when they resettle
they actually become more dense adding to the weight around rooftop
units, Wistar said. RELATED:AccuWeather Winter Weather CenterExperts debunk 4 winter driving mythsHow to survive a power outage in winter
“When snow weighs 10 pounds per
square foot that is okay, but when it starts getting into the 20s then
it can be a concern for a roof,” Wistar said.
Over time snow
compacts and settles down, meaning the snow won’t be as deep but the
weight will be the same. What was once one foot of snow may become less,
but the weight will not change, Wistar said.
And when
temperatures rise and snowfall turns to rainfall, the snow already
coating rooftops can become saturated and heavy with rain.
When
the snow does begin to melt away, it can refreeze around gutters and
drains under lower temperatures, trapping more melting water on the
roof. The trapped water can not only leak into the home's interior, but
it causes further stress on the roof.
Different roof designs are built to cope with snow differently, each one with both advantages and disadvantages.

(iStock/Thinkstock)

Sloped roofs
Gable
roofs, which have two sloping sides, can become problematic in heavy
wind conditions, when snow will blow from one side and settle on the
other.
Some sloped roofs, with enough slope and that are short enough, can allow snow to blow from one side then onto the ground.
But sloped roofs are also prone to ice dams, which can cause interior and roof damage.
No
house insulation is perfect and as heat escapes up through the middle
of the roof, it can melt away the lower layers of the snowpack. As this
water migrates away from the heated center and reaches the cold eaves,
or edges, of the house it can quickly freeze, Wistar said.
Other
variables such as solar radiation and the home's orientation toward
sunlight can impact how prone the house is to ice dams. The snow will
melt in the sun but refreeze in the shade.

A car drives by Schmidt's
Collision and Glass after the roof collapsed after taking on heavy snow
during this week's lake-effect snowstorms on Saturday, Nov. 22, 2014,
in Hamburg, New York. (AP Photo/Mike Groll)

An ice dam can cause damage to the
drainage system and gutters as it prevents other water run-off from
escaping. As water is backed up behind the thick dam, it can infiltrate
into the home’s interior and keep unwanted weight on top of the roof.
“When
you have an ice dam, you block water from going anywhere and it can
sink back under the roof,” said Wister. “And anywhere that water
collects and freezes, the solid ice is going to be heavier.”
This heavy ice can also cause stress and damage to the roof, Wistar said.
A-frame roofs, which have very steep slopes, speed up water run-off so that it doesn't have the opportunity to refreeze.
However, these sloped roofs typically aren’t feasible for large industrial buildings, hospitals or any other large building.

(iStock/Thinkstock)

Flat roofs
While most
roofs are not entirely flat, roofs with a level appearance allow snow to
blow off the roof easily. However, some of these roofs have external
systems on the roof, including air conditioning and parapet units, which
encourage snow drifts to settle around the unit.
These roofs are
also less prone to ice dam formation, but the draining systems on top
of the roof can freeze over trapping melting water on the roof.
Wistar
said that there really is no ideal roof type to battle winter weather,
and it all really comes down to the type of snow and the amount of wind.
For more safety and preparedness tips, visit AccuWeather.com/Ready.